1. Field of the Invention
The present invention relates to a valve lifter, and to a method of manufacturing the same. Specifically, the present invention relates to a valve lifter, and a method of making the valve lifter, in which a cam-contacting sliding surface is concave in shape, and is formed in an upper surface of the valve lifter.
2. Description of the Background Art
Valve lifters are generally well known in the art of internal combustion engines, and have been conventionally used for many years. A valve lifter, as a component of a valve train of an internal combustion engine, includes a sliding surface on which a cam slides repeatedly during engine operation. The sliding surface is exposed to severe sliding conditions, and measures for improving durability and improved cam sliding properties on the sliding surface have long been studied. An effective measure for improving lubricating properties on the cam-contacting sliding surface has been especially required. In these circumstances, a shimmed valve lifter is known as a measure for improving the cam sliding properties by improving the lubricating properties on the cam-contacting sliding surface, for example, in which a shim is disposed in an upper surface of the valve lifer, forming a sliding surface on which a cam slides. In this known lifter, the upper surface of the shim is formed having a concave portion thereon. This is disclosed, for example, in Japanese Patent Laid-open Publication No. 9-195723 (p. 3 to 4, FIG. 1). FIG. 4 of the drawings in the present application is a reproduction of a drawing from Japanese Patent Laid-open Publication No. 9-195723, and is included herein for purposes of illustration and comparison.
In the lifter design of Japanese Patent Laid-open Publication No. 9-195723, the cam 06, including a cam surface 07 with a linear cross section, is brought into sliding contact with the shim 04 at a position offset from the center of the valve lifter. Rotational speed of the shim 04 is therefore increased, thus theoretically improving the capability of lubrication oil to form an oil film therebetween.
As shown in FIG. 4, the valve lifter described in the above Japanese Patent Laid-open Publication No. 9-195723 includes a disk-shaped shim 04 having a sliding surface 05, on which a cam slides, on the upper surface 02 of the valve lifter 01. The shim 04 is fit loosely in a socket formed in the upper surface 02 of the lifter, as shown, to enable the shim to rotate relative to the upper surface 02 of the valve lifter 01 during engine operation. The shim 04 is held in place by an annular protruding wall 03 provided in the outer periphery of the upper surface 02 of the valve lifter 01 so as to define a socket for housing the shim, and prevent the valve lifter 01 from being disengaged. The upper surface 05 of the disk-shaped shim 04 is a concavely curved sliding surface on which the cam slides. The cam 06, which is brought into sliding contact with the sliding surface 05, includes a cam surface 07 with a linear cross section. Moreover, the cam 06 is provided to be disposed at a position axially offset relative to a center axis 0X-0X of the shim 04 and valve lifer 01 (see a cam center line 0Y-0Y).
Accordingly, the cam 06 is brought into sliding contact with the valve lifter 01 at a position that is offset outwardly relative to the center position, that is, the lowest position in the substantially concavely curved sliding surface 05 of the disk-shaped shim 04. Furthermore, the cam 06 includes the cam surface 07, which has a linear cross section. The cam surface 07 is brought into sliding contact with the concavely curved sliding surface 05 at a corner portion 08 of the cam lobe, that is close to the outer periphery. This speeds up the rotation of the shim 04 accompanied with rotation of the cam 06 to increase the rate of lubricating oil being drawn onto the sliding surface 05, which is a portion where the cam surface 07 and the shim 04 are brought into sliding contact with each other, thus improving the capability of lubricating oil to form an oil film.
However, in this known design, providing the above-described shim in the upper surface of the valve lifer increases the weight and complexity of the valve lifter assembly. The weight increase is opposed to a goal of weight reduction of the valve lifter. Moreover, the weight increase of the valve lifter increases inertia of the valve lifter, thus degrading an ability of the valve lifter to follow the valve opening and closing in high-speed operations. Furthermore, the shim, having a special shape, is formed as a separate body from the body of the valve lifer, complicating the structure. Moreover, the number of man-hours required to manufacture the valve lifter is increased, and accordingly, the manufacturing cost increases.
As described above, the concave sliding surface stabilizes the rotation of the valve lifter and facilitates retaining lubricating oil on the sliding surface. However, on the other hand, there is a problem of lower resistance to wear and scuff in a severe operating situation, since the surface pressure increases in the outer portion which is brought into contact with the cam. The concave valve lifter, therefore, has not been put into significant effective practical use heretofore.